Molecular Biology of the Platelet Aggregation-Associated Protein of Streptococcus sanguis) Certain strains of viridans streptococci (Agg+), including the predominant member of the oral commensal flora. Streptococcus sanguis, induce human platelets to aggregate in vitro. When viridans streptococci entered the body, they may become significant pathogens or even etiological agents of certain severe human diseases by interacting with platelets, such as bacterial endocarditis, Beheet's syndrome, etc. Three different surface antigens of S. sanguis (Adh+, Agg+) have been proposed to be involved in the interaction with platelets, which are class I antigen (adhesin), class II antigen, which is also called platelet-aggregation associated protein (PAAP), and class III antigen, which has ecto-ATPase activity. S. sanguis cells bind to platelets by class I antigen, and then class II antigen triggers and activates the release of the dense granules and the aggregation of platelets, the class III antigen amplifies the reaction by hydrolyzing ATP released from dense granules into ADP. While in search of the gene of the PAAP from genome of S. sanguis, a gene encoding a 65kDa heat shock protein (HSP65) was discovered, which is a homolog of the groEL gene of E. coli and hsp65 gene of M. tuberculosis. Sequence data from the hsp65 gene of S. sanguis indicated high homology also to human PI protein gene, a human homolog of hsp65. Purification of the 65Kda HSP from S. sanguis was conducted. The final identity of the protein was confirmed by internal peptide sequences. To clarify the differences between HSP65 of S. sanguis and PAAP, platelet aggregation inhibition assays were carried out with the HSP65 of S. sanguis. Platelet aggregation was inhibited when the platelets preincubated with the HSP65. However, the HSP65 seems less potent with regard to its ability to inhibite platelet aggregation than PAAP. Another significant difference between these two proteins is that the HSP65 does not contain the platelet interactive domain as in the PAAP and collagen. In conclusion, the identification of the HSP65 from S. sanguis may provide a useful tool to study antoimmune diseases. This protein is different from PAAP which has been identified from this lab previously. Key words: genes, cloning, sequencing. Streptococcus sanguis, platelet: